1. Field of the Invention
The present invention relates to an optical communication system based on optical modulation using a microwave frequency-division multiplexed signal and, more particularly, to an SCM (subcarrier multiplexed) optical communication system.
The SCM optical communication system has an excellent feature in that all kinds of signals in either analog or digital form can be transmitted at the same time and in large quantities using only a single optical carrier. The present invention employs FM modulation based on direct modulation by a semiconductor laser, etc., as the optical modulation in the SCM optical communication system. The present invention can be applied to all types of information communication networks including a conventional optical communication network, an optical CATV network, a broadband distribution network mainly handling image information, and future ISDN (integrated services digital network).
2. Description of the Related Art
Heretofore, wavelength-division multiplexed transmission has been used mainly for multiplexing transmitting signals in optical communication (such as large-capacity signal multiplexed transmission, in particular). This is intended for multi-channel transmission with optical waves of different wavelengths (or frequencies) used as carriers. In the case of an intensity modulation and direct detection (IM/DM) system, the wavelength-division multiplexing transmission requires a channel spacing of the order of several nanometers in wavelength so that channels can be distinguished from each other by optical filters. In the case of a coherent optical communication system, a channel spacing is presently required which is of the order of ten and several times the bit rate because the ability to control crosstalk between adjacent channels is limited. In the case of, for example, high-speed multiplexed transmission at a bit rate in the order of gigabits, the channel spacing is about 20 GHz. A receiver which can detect all of the channels simultaneously cannot be realized. However, the number of channels to be multiplexed may be increased at the sending end. Therefore, only one channel can be received at a time.
On the other hand, the conventional SCM optical communication system uses mainly an intensity modulation system based on the direct modulation of a semiconductor laser as its optical modulation system, and a direct detection system using a PIN photodiode or APD as its receiving system.
By way of example, a conventional FDM (frequency-division multiplexed) optical transmission system and a conventional TDM (time-division multiplexed) optical transmission system are illustrated in FIG. 1 and FIG. 2, respectively. In both systems, coherent transmission of four-channel, 622 Mb/s signals is made, and the optical transmission capacity is 2.5 Gb/s.
In the optical FDM system, as can be seen from FIG. 1, optical modulators 1--1 to 1-4, which correspond in number (four in this example) to channels and have optical carriers of different frequencies fs1 to fs4, each produce a modulated optical signal. These modulated signals from the optical modulators are mixed in an optical coupler 2 to produce a frequency-division multiplexed optical signal which, in turn, is transmitted through an optical fiber 3. At the receiving end, the optical signal transmitted through the optical fiber is mixed with a local light source (semiconductor laser) and heterodyne detected by means of a local oscillator, an optical receiver 5 and an amplifier 6, whereby it is converted to an electric intermediate frequency signal. The resulting electric signal is then filtered by a bandpass filter 7 to allow only the signal on a desired channel to pass. The signal is then demodulated by a demodulator 8.
In the TDM optical transmission system, as can be seen from FIG. 2, signals transmitted on channels are time-division multiplexed by a multiplexer (MUX) 11 to produce a time-division multiplexed signal which, in turn, modulates an optical modulator 12. The modulated signal is transmitted through an optical transmission fiber 13. At the receiving end, the optical signal transmitted from the transmitting end is optical-heterodyne detected by the use of a local light source (semiconductor laser) 14, an optical receiver 15 and an amplifier 16 for conversion to an electric signal. The resulting electric signal is filtered by a bandpass filter 17 to pass intermediate frequency signals of all the channels (bandwidth is about 25 GHz.times.2). These signals are demodulated by a demodulator 18 and then separated by a demultiplexer (DEMUX) 19 into the signals for the respective channels.
A problem with such a conventional optical frequency-division multiplex transmission system as shown in FIG. 1 is that, since the channel spacing at the time of signal multiplexing must be large, of the order of ten and several times the bit rate, multiplexed signals cannot be detected simultaneously at the receiving end. However, the number of channels to be multiplexed may be increased at the transmitting end, but a limited number of channels that can actually be detected. In the case of high-speed transmission of digital data (gigabit transmission) in particular, only one channel can be detected at a time by a receiver. Also, the number of channels which can be multiplexed is limited by the band of frequencies over which a semiconductor laser serving as a local light source is tunable.
An optical time-division multiplex transmission system such as that shown in FIG. 2 requires a multiplexer for time-division multiplexing and a demultiplexer for separating the components in a time-division multiplexed signal transmitted over an optical transmission line. These circuits are very expensive. The use of these circuits will lead to an increase of cost of the whole system. Moreover, the heterodyne detector requires a demodulation bandwidth which is wide enough to cover all the channels that are multiplexed.
The conventional SCM optical communication system described above requires that the light output be linear with the intensity modulation of the semiconductor laser. This sets a limit to the bandwidth for modulation. Under the present circumstances, the bandwidth is in the order of 1 to 2 GHz at best. Thus, the wideband transmission is apt to be influenced by signal distortion, thus limiting the capacity of transmission information. This makes it difficult to meet the requirements of future large capacity communications. Moreover, since the direct detection system alone can be used as the detection system, it is difficult to achieve a sufficient receiver sensitivity. Therefore, there are limits to transmission distance and the number of signals to be distributed.
If, therefore, optical angular modulation could be used as the optical modulation in the SCM optical communication system, such technical problems would be solved. In this case, however, to achieve a high receiver sensitivity, the coherent optical transmission system must be used, which requires an optical heterodyne receiver or an optical homodyne receiver, which have complex structures. It also requires countermeasures against degradation of the receiver sensitivity due to fluctuations in the state of polarization of signal light and local light. This will make the receiver very expensive. It is difficult to use such an expensive receiver in subscriber systems.